U.S. patent application number 10/233870 was filed with the patent office on 2004-03-18 for ultrasonic imaging devices and methods of fabrication.
Invention is credited to Stephens, Douglas Neil.
Application Number | 20040054287 10/233870 |
Document ID | / |
Family ID | 31977312 |
Filed Date | 2004-03-18 |
United States Patent
Application |
20040054287 |
Kind Code |
A1 |
Stephens, Douglas Neil |
March 18, 2004 |
Ultrasonic imaging devices and methods of fabrication
Abstract
A sensor for an ultrasound imaging catheter and methods of
fabrication are provided. The sensor may be based on a flex circuit
on which a block of piezoelectric sensor array transducer material
is mounted. The flex circuit may include electrical conductors that
are electrically connected to electrodes on the piezoelectric
blocks. A matching layer may be formed on the piezoelectric blocks
between the blocks and the flex circuit substrate. Individual
transducer array elements may be formed by dividing a piezoelectric
block into a plurality of individual transducer elements after the
matching layer has been formed. Cuts may be formed in the flex
circuit substrate between adjacent transducer array elements to
acoustically decouple adjacent elements. The flex circuit substrate
and matching layers may have relatively high impedances to
facilitate acoustic impedance matching between the sensor and the
imaging environment.
Inventors: |
Stephens, Douglas Neil;
(Davis, CA) |
Correspondence
Address: |
FISH & NEAVE
1251 AVENUE OF THE AMERICAS
50TH FLOOR
NEW YORK
NY
10020-1105
US
|
Family ID: |
31977312 |
Appl. No.: |
10/233870 |
Filed: |
August 29, 2002 |
Current U.S.
Class: |
600/459 |
Current CPC
Class: |
G01N 2291/106 20130101;
A61B 8/4483 20130101; G01N 29/2437 20130101; A61B 8/12 20130101;
B06B 1/0622 20130101; A61B 8/4488 20130101; G01N 29/2468
20130101 |
Class at
Publication: |
600/459 |
International
Class: |
A61B 008/14 |
Claims
The invention claimed is:
1. A sensor for(an ultrasonic imaging device operated in a medium
having an acoustic impedance, comprising: a flex circuit having a
flex circuit substrate and a plurality of electrical conductors
formed on the flex circuit substrate; and an array of individual
piezoelectric transducer array elements arranged around the flex
circuit, wherein the transducer array elements have
simultaneously-formed acoustic matching layer portions that help to
match the acoustic impedance of the transducer array elements to
the acoustic impedance of the medium, wherein the
simultaneously-formed acoustic matching layer portions are disposed
between the transducer array elements and the flex circuit
substrate.
2. The sensor defined in claim 1 wherein the matching layer
comprises a material having an acoustic impedance in the range of
5-12 MRayls.
3. The sensor defined in claim 1 wherein the flex circuit substrate
has an acoustic impedance in the range of 3.5-4.5 MRayls.
4. The sensor defined in claim 1 wherein the transducer array
elements are acoustically decoupled from each other by cuts formed
through the flex circuit substrate between adjacent transducer
array elements.
5. The sensor defined in claim 1 wherein a plurality of integrated
circuits are mounted on the flex circuit.
6. The sensor defined in claim 1 wherein the flex circuit substrate
comprises a flexible film having an acoustic impedance of at least
3.5 MRayls and the transducer array elements are acoustically
decoupled from each other by cuts formed through the flex circuit
substrate between adjacent transducer array elements.
7. The sensor defined in claim 1 wherein there are between 32 and
128 transducer array elements mounted to the flex circuit.
8. The sensor defined in claim 1 wherein there are electrodes on
each of the piezoelectric transducer array elements, and wherein
the electrodes are connected to the electrical conductors on the
flex circuit using conductive fillets.
9. An ultrasonic imaging catheter sensor, comprising: a flex
circuit having a flex circuit substrate, wherein there are a
plurality of electrical conductors on the flex circuit substrate;
and an array of individual piezoelectric transducer array elements
arranged around the flex circuit, wherein the transducer array
elements are separated by longitudinal cuts in the flex circuit
substrate between adjacent transducer array elements.
10. The sensor defined in claim 9 wherein the transducer array
elements are separated by kerfs and wherein the cuts are extensions
of the kerfs.
11. The sensor defined in claim 9 wherein the flex circuit
substrate comprises a material having an acoustic impedance in the
range of 3.5-4.5 MRayls.
12. The sensor defined in claim 9 further comprising a matching
layer of a material having an acoustic impedance in the range of
5-12 MRayls, wherein the matching layer is disposed on the
transducer array elements between the transducer array elements and
the substrate of the flex circuit.
13. A method of forming a flex-circuit sensor having multiple
piezoelectric transducer array elements for an ultrasound imaging
catheter, comprising: placing a plurality of piezoelectric blocks
in a template; heating the template to cause the template to flow
and cover the sides of the piezoelectric blocks; forming a matching
layer of a material having an acoustic impedance of 5-12 MRayls on
the piezoelectric blocks after the sides have been covered;
mounting one of the piezoelectric blocks on a flex circuit so that
the matching layer is between the piezoelectric block and the flex
circuit; and dividing the mounted piezoelectric block into
individual piezoelectric transducer array elements.
14. The method defined in claim 13 further comprising shaping the
flex circuit into a cylinder so that the individual piezoelectric
transducer array elements form a substantially cylindrical
ultrasound sensor array.
15. The method defined in claim 13 further comprising covering the
tops of the piezoelectric blocks with a flexible cover to prevent
the template from coating the tops of the piezoelectric blocks
during heating.
16. The method defined in claim 13 further comprising polishing the
matching layer.
17. The method defined in claim 13 further comprising forming cuts
through the flex circuit between adjacent piezoelectric transducer
elements.
18. The method defined in claim 13 further comprising: placing a
temporary stabilizing layer of material on the flex circuit before
the formation of cuts through the flex circuit between adjacent
piezoelectric transducer elements; forming the cuts through the
flex circuit between adjacent piezoelectric transducer elements;
and removing the temporary stabilizing layer after the cuts have
been formed.
19. The method defined in claim 18 wherein the temporary
stabilizing layer of material comprises a photoresist layer, the
method further comprising holding the photoresist layer using a
vacuum chuck.
20. The method defined in claim 13 further comprising mounting the
piezoelectric block on a flex circuit having an acoustic impedance
in the range of 3.5-4.5 MRayls.
21. The method defined in claim 13 wherein forming the matching
layer comprises forming a matching layer having an acoustic
impedance of 6-8 MRayls on the piezoelectric blocks.
Description
BACKGROUND OF THE INVENTION
[0001] This application relates to ultrasonic imaging devices such
as ultrasonic imaging catheters and sensors and to methods for
fabricating these devices.
[0002] Ultrasonic imaging techniques are often used to gather
images during the diagnosis and treatment of medical conditions. An
ultrasonic imaging catheter may be used to gather images from
within a patient's body. During percutaneous transluminal coronary
angioplasty procedures, for example, images may be acquired from
within the blood vessels of a cardiac patient to help a physician
to accurately place an expandable balloon.
[0003] In a typical ultrasound imaging catheter configuration, a
piezoelectric ultrasound transducer array at the distal end of the
catheter may be used to generate high-frequency acoustic signals
that radiate towards the image target (e.g., a patient's blood
vessel). The transducer array gathers corresponding reflected
acoustic signals. Image processing techniques are used to convert
the reflected acoustic signals into images for the physician.
[0004] It is an object of the present invention to provide improved
ultrasonic imaging catheters and sensors and methods for
fabricating such devices.
SUMMARY OF THE INVENTION
[0005] This and other objects of the invention are accomplished in
accordance with the principles of the invention by providing
ultrasonic imaging catheters and sensors and methods for their
fabrication.
[0006] An imaging catheter constructed in accordance with the
invention may have a sensor at its distal tip. The sensor may have
a transducer array formed from piezoelectric elements. During the
manufacturing process, a uniform acoustic matching layer may be
formed over the surface of all of the elements in the transducer
array in parallel. These simultaneously-formed acoustic matching
layer portions on the transducer array elements improve the
performance of the transducer array when the imaging catheter is
used to gather images of a suitable image target (e.g., the blood
vessels or other body lumens of a patient). Specifically, the
matching layer matches the acoustic impedance of the transducer
elements to the surrounding medium (e.g., blood, tissue, etc.) by
serving as an intermediate layer of intermediate impedance and
suitable thickness. Thus, the matching layer may be formed using a
material that has an acoustic impedance between that of the
transducer elements and the body fluid or other substance in which
the sensor is immersed during operation.
[0007] The sensor may have a flexible circuit ("flex circuit") on
which the transducer array elements are formed. The flex circuit
substrate may help to match the acoustic impedance of the
piezoelectric transducer elements to the acoustic impedance of the
medium in which the sensor is immersed. To enhance the acoustic
matching capabilities of the flex circuit, the flex circuit
substrate may be formed using a flexible substrate material with a
relatively high acoustic impedance for flexible polymeric
materials. As an example, within the polyimide group of materials,
Upilex has a higher acoustic impedance than Kapton. The flex
circuit may have conductors to which the transducer array elements
are electrically connected.
[0008] The transducer array elements may be mounted to the flex
circuit in the form of a block of piezoelectric material that is
subsequently divided to form the individual transducer array
elements. During the process of dividing the transducer array into
individual elements, the flex circuit that underlies the transducer
array may be divided as well, so as to acoustically decouple the
transducer array elements from adjacent elements. This is expected
to improve imaging performance by reducing cross-talk.
[0009] Further features of the invention, its nature and various
advantages will be more apparent from the accompanying drawings and
the following detailed description of the preferred
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a perspective view of an illustrative ultrasonic
imaging catheter and accompanying image processing equipment in
accordance with the present invention.
[0011] FIG. 2 is a perspective view of the distal tip of an
illustrative support lumen that may be used in the core of an
ultrasonic imaging catheter device in accordance with the present
invention.
[0012] FIG. 3 is a perspective view of the distal tip of another
illustrative support lumen that may be used in the core of an
ultrasonic imaging catheter device in accordance with the present
invention.
[0013] FIG. 4 is a perspective view of a transducer array and
integrated circuits that have been formed on a flex circuit in
accordance with the present invention.
[0014] FIG. 5 is a cross-sectional side view of a portion of an
illustrative ultrasonic imaging catheter in the vicinity of the
transducer array and accompanying integrated circuits in accordance
with the present invention.
[0015] FIG. 6 is a cross-sectional view of the illustrative
catheter of FIG. 5 taken along the line 6-6 in accordance with the
present invention.
[0016] FIG. 7 is a perspective view of an illustrative transducer
array element showing the placement of the transducer electrodes in
accordance with the present invention.
[0017] FIG. 8a is a cross-sectional end view of the transducer
array element of FIG. 7 mounted in an illustrative fashion to a
flex circuit in accordance with the present invention.
[0018] FIG. 8b is a perspective view of a portion of a flex circuit
and piezoelectric transducer prior to using a conductive fillet to
connect the transducer electrodes to electrical conductors on the
flex circuit in accordance with the present invention.
[0019] FIG. 8c is a perspective view of a portion of a flex circuit
and piezoelectric transducer after using a conductive fillet to
connect the transducer electrodes to electrical conductors on the
flex circuit in accordance with the present invention.
[0020] FIG. 9 is a cross-sectional view (in perspective) of an
ultrasonic transducer array having multiple elements of the types
shown in FIG. 7 and 8a after mounting to a flex circuit in
accordance with the present invention.
[0021] FIGS. 10 and 11 are schematic views of illustrative template
structures that may be used when fabricating transducer arrays in
accordance with the present invention.
[0022] FIG. 12 is a side view of an illustrative template structure
prior to insertion of a block of piezoelectric material for
processing to form a transducer array in accordance with the
present invention.
[0023] FIG. 13 is a side view similar to that of FIG. 12 showing
how a piezoelectric block may be inserted into the template during
the process of fabricating the transducer array in accordance with
the present invention.
[0024] FIG. 14 is a side view of the template and piezoelectric
block of FIG. 13 after the block and template have been covered by
a flexible cover in accordance with the present invention.
[0025] FIG. 15 is a side view of the template structure after the
flexible sheet of FIG. 14 has been removed in accordance with the
present invention.
[0026] FIG. 16 is a side view of the template structure of FIG. 15
after a layer of acoustically matching material has been deposited
on the piezoelectric block and planarized in accordance with the
present invention.
[0027] FIG. 17 is a side view of the piezoelectric block after
removal from the template structure of FIG. 16 in accordance with
the present invention.
[0028] FIG. 18 is a flow chart of steps involved in fabricating an
ultrasonic imaging sensor and catheter in accordance with the
present invention.
[0029] FIG. 19 is a perspective view showing the end of a portion
of an illustrative sensor in which transducer array elements have
been acoustically decoupled from each other by forming cuts through
flex circuit portions between adjacent elements in accordance with
the present invention.
[0030] FIG. 20 is a perspective view of the distal portion of an
illustrative ultrasonic imaging catheter showing (from an exterior
perspective) the location of cuts made to acoustically isolate
individual transducer array elements from each other in accordance
with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] An illustrative catheter-based ultrasonic imaging system 10
in accordance with the invention is shown in FIG. 1. A catheter 12
with a ultrasonic sensor 16 may be used to gather images from
locations inside a patient's body during surgical and diagnostic
procedures. For example, catheter 12 may be used during
percutaneous procedures such as cardiac surgery to gather images
from inside a patient's blood vessels. The catheter may be used for
other ultrasound applications if desired.
[0032] The catheter may have any suitable dimensions. For example,
the catheter may have an overall length of about 100-200 cm. The
distal portion of the catheter may have a length of about 35 cm and
the proximal portion of the catheter may have a length of about 125
cm.
[0033] Image data from sensor 16 may be provided to image
processing and display equipment 14 using cables within catheter
12. Equipment 14 may use ultrasound image processing techniques to
process the image data and to display corresponding images on a
display screen for a physician or other suitable user. Equipment 14
provides signals to sensor 16 that control the operation of sensor
16. For example, equipment 14 may provide drive signals for the
transducer elements in sensor 16 that cause those elements to emit
appropriate acoustic signals into the surrounding area of the
patient's body. Appropriate acoustic signals may, for example, be
acoustic pulses of approximately 25 ns in duration.
[0034] The peak drive voltage that is impressed upon the
piezoelectric transducer elements to generate these pulses may be
about 10 V. The acoustic echoes from the patient's body may induce
voltages in the transducer elements on the order of 10-30 .mu.V.
Because the induced signals from reflections are typically several
orders of magnitude lower than the drive signal, the acoustic
impedance of the sensor is preferably matched to the surrounding
environment. Individual transducer elements may also be
acoustically isolated from each other to improve imaging
performance.
[0035] Sensor 16 may be mounted on any suitable medical device such
as a catheter of the type shown in FIG. 1 or a probe. For example,
sensor 16 may be mounted on a catheter that contains a hollow tube.
A guide wire that runs through the hollow tube may be used to
assist in the placement of the catheter at a desired location
within the patient's body. In another suitable arrangement, sensor
16 may be affixed to the end of a probe without a hollow tube that
is manipulated at its proximal end by a physician or other suitable
user. For clarity, the present invention will be describe primarily
in the context of sensor arrangements involving catheters. This is,
however, merely illustrative. Sensor 16 may be used with any
suitable intraluminal and/or medical instrument or equipment.
[0036] Sensor 16 may be permanently affixed to a medical instrument
or may be provided as part of a separate structural element that is
removable from the instrument. For example, sensor 16 may be
provided as part of an interchangeable catheter tip.
[0037] Additional components may be provided on catheter 12 if
desired. For example, fiber-optic cables may be used to provide
video imaging capabilities, expandable balloons may be used to
deploy stents, atherectomy tools may be used to clear blockages,
snares or probe tips may be provided for use during surgery,
irrigation ports or guide wire passages may be provided, and
temperature sensors, flow sensors (e.g., Doppler flow sensors),
pressure sensors, and combinations of such sensors may be provided,
etc. These are merely illustrative examples. Any suitable
components may be included in catheter 12 if desired.
[0038] Sensor 16 may include a transducer array based on a number
of individual transducer elements. Any suitable number of elements
may be used (e.g., 16, 32, 64, 80, 128, 256, more than 256, less
than 16, 32-256, etc.). The transducer elements may be formed of a
piezoelectric material (e.g., a ceramic piezoelectric material, a
polymeric piezoelectric material, a single crystal piezoelectric
material, a composite piezoelectric material having ceramic
particles in a matrix, etc.). When driven by suitable AC electrical
signals, the transducer elements produce acoustic waves that
radiate into the patient's body. These transducer elements (or a
separate set of receiver transducer elements) are used to gather
reflected acoustic waves and to convert those signals into acoustic
energy.
[0039] In one suitable arrangement, sensor 16 includes a number of
transducer elements that are formed on a flexible substrate that
includes conductive electrodes for carrying electrical signals to
and from the transducer elements. This type of substrate, which is
referred to as a "flex circuit," is relatively thin (e.g., 5 .mu.m
to 200 .mu.m thick or any other suitable thickness) and may be
rolled up or otherwise manipulated to form a cylinder that is
affixed to the cylindrical core of the catheter 12 at an
appropriate distal location as shown in FIG. 1. The flex circuit
portion may be on the outermost portion of the sensor, so that
during operation acoustic energy from the transducer array elements
radiates radially outward through the flex circuit into the
patient's body. The outer diameter of the flex circuit after it has
been attached to the catheter may be on the order of 0.5 mm to 3 mm
or any other suitable diameter.
[0040] It may be desirable to place an acoustic backing layer
between the catheter core and the transducer array elements to
reduce acoustic interference in the form of spurious "ringing"
signals reflected from the core into the transducer elements. The
backing layer and rolled-up cylindrical transducer array may be
mounted in a groove in the catheter core. Suitable catheter
arrangements having a groove for accommodating the backing layer
material and cylindrical sensor elements are shown in FIGS. 2 and
3.
[0041] In the arrangement of FIG. 2, catheter 12 has a main
cylindrical core or body portion formed from a hollow or solid tube
or a series of tubes or tube sections or other individual parts.
The catheter core may be formed using metal, plastic, a combination
of metal and plastic parts, or any other suitable material or
combination of materials. All or a portion of the catheter (e.g.,
in the vicinity of the catheter tip) may be formed using a
radiopaque material, e.g., a material that is visible during x-ray
fluoroscopy, such as a metal alloy containing platinum or iridium.
A typical hollow tube may have a diameter of approximately 0.024
inches and a wall thickness of 25 .mu.m. At the distal tip of
catheter 12, the elements of core 18 may be shaped to form a
circumferential groove 20. Groove 20 is preferably of sufficient
depth to accommodate all or at least some of the thickness of the
backing layer material and the transducer array elements when the
catheter is fully assembled. In the illustrative arrangement of
FIG. 2, groove 20 is formed by forming an indentation with the
components of catheter core 18 between a distal portion attached to
the main length of the catheter and a tip portion 22. Tip 22 may be
formed from the same material or materials as catheter core 18 or
may be formed from a soft material that, in combination with the
tapered shape of tip 22, facilitates insertion of catheter 12 into
narrow lumens such as blood vessels. A guide wire lumen 24 may be
provided in catheter 12 so that catheter 12 may be advanced over
guide wire 26.
[0042] In the illustrative arrangement of FIG. 3, annular rings 22
have been used to form spacers that define a groove 20. Rings 22
may be formed of any suitable material. For example, rings 22 may
be formed from rubies, which advantageously may be machined to
tight tolerances. Other suitable materials that may be used include
plastic, ceramic, metal, epoxy, composites of such materials, etc.
Rings 22 may be affixed to core 18 using an adhesive such as a
cyanoacrylate adhesive. Cyanoacrylate adhesive may also be used to
affix other portions of core 18 to each other. For example, when
core 18 is formed from nested or overlapping tubes, cyranoacrylate
adhesive may be used to secure the tubes to each other.
[0043] As shown in FIGS. 2 and 3, a guide wire lumen 24 that runs
longitudinally through the center of catheter 12 may be provided.
This allows catheter 12 to be guided over a guide wire 26, which
facilitates placement of catheter 12 during use in a patient.
[0044] The arrangements for defining the groove 20 on catheter core
18 that are shown in FIGS. 2 and 3 are merely illustrative. Any
suitable arrangement may be used to define groove 20 if desired.
The groove may be defined by forming an indentation in the tube or
tubes or cylinder of material that is used to form the main body of
catheter 12, may be formed by adding additional spacers, rings or
other structures over the outer circumference of the core 18, may
be formed by attaching a specially formed tip having an integral
groove portion, or may be formed using other suitable approaches or
combinations of such approaches.
[0045] As shown in FIGS. 2 and 3, groove 20 may have a length L (as
measured along the longitudinal axis of catheter 12). The length L
may be sufficient to accommodate the transducer array when the
transducer array and other components on the flex circuit are
rolled up to form a cylinder around the distal end of catheter 12.
For example, if the corresponding dimension of the transducer array
is 1 mm, then length L should be 1 mm plus a small clearance to
allow the transducer array to be mounted in groove 20.
[0046] An illustrative sensor 16 having a transducer array 28 with
a dimension L for mounting in a groove 20 of the type shown in
FIGS. 2 and 3 is shown in FIG. 4. Transducer array 28 may have a
number of individual elements, each of which is aligned in parallel
with the illustrative element 30 shown in FIG. 4. The transducer
array 28 is mounted on a flex circuit 32. The flex circuit may be
formed from a flexible substrate material such as polyimide, which
is electrically insulating. If desired, the flex circuit may be
formed from a substance having a relatively high acoustical
impedance for flexible polymeric materials. Within the polyimide
group of materials, Upilex has a higher acoustic impedance than
Kapton. With one suitable arrangement, the flex circuit may be
formed form a substance that is flexible and that has an acoustic
impedance of at least 3.5 MRayls or an acoustic impedance in the
range of 3.5 to 4.5 MRayls. A suitable flex circuit substrate
material that has an acoustic impedance of about 4 MRayls is
available under the trade name Upilex-S, from Ube Industries, Inc.
of Yamaguchi, Japan. Using a flex circuit with an acoustic
impedance of 3.5 to 4.5 MRayls may significantly improve the
acoustic impedance matching between sensor 16 and the medium in
which catheter 12 is immersed (typically a patient's blood). Such
acoustic impedance matching is expected to improve the performance
of the sensor 16.
[0047] Electrical conductors 34 are formed on the surface of the
flex circuit substrate. The electrical conductors may be formed,
for example, from a malleable metal such as gold. A suitable
adhesion layer such as a thin layer of chromium may be used to
facilitate adhesion of the gold or other conductor material to the
substrate. Metal layers may be deposited by sputtering,
evaporation, or any other suitable technique. Wet or dry etching or
other suitable patterning techniques may be used to pattern the
deposited metal to form electrical conductors 34.
[0048] Each transducer element 30 may have two opposing electrodes.
The main portion of the electrodes is located on the upper and
lower surfaces of the transducer array when the array is oriented
as shown in FIG. 4. Smaller portions of the electrodes extend over
the ends 35 and 36 of the elements 30 in transducer array 28.
Electrical signals may be conducted between the conductors 34 and
the main portions of the electrodes by forming electrical contacts
between the conductors 34 and the end portions 35 and 36.
[0049] By connecting the electrodes on each transducer element 30
to corresponding conductors 34, drive signals for the transducer
elements 30 may be conveyed to the elements 30. Similarly,
electrical signals that are produced by the elements 30 when
reflected acoustic waves are detected by elements 30 may be
conveyed from the elements.
[0050] In some transducer arrays (e.g., arrays with 64 elements or
more), there may be so many conductors 34 that it is cumbersome to
route all of these conductor lines to equipment 14 in a single
cable along the length of the catheter 12. Accordingly, electrical
multiplexer circuits 38 (e.g., time-division multiplexing circuits
or other suitable multiplexing circuits) may be used to reduce the
relatively large number of conductors 34 that are directly
connected to transducer array 28 into a smaller number of
conductors 34 at the input/output 40. The conductors at
input/output 40 may be soldered, welded, or otherwise electrically
connected to wires in a suitable cable that runs along the length
of catheter 12 to equipment 14. If desired, circuits 38 may include
drive circuitry for generating drive signals and/or preprocessing
circuitry for at least partially processing the electrical signals
that are produced when the transducer elements 30 in array 28 are
used to detect acoustical information.
[0051] After circuits 38 and transducer array 28 have been mounted
on flex circuit 32, as shown in FIG. 4, the flex circuit and
mounted components may be formed into a cylindrical shape and
attached to the distal section of catheter 12, so that array 28
protrudes into groove 20 (see FIGS. 2 and 3). A cross-sectional
side view of an illustrative catheter 12 after the flex circuit and
components of FIG. 4 have been attached to the catheter is shown in
FIG. 5.
[0052] As shown in FIG. 5, the ends 35 and 36 of array 28 are held
in place between the inner ends of groove 20 (defined by the
circumferential inner end faces of annular spacer rings 22 in the
embodiment of FIG. 5). Integrated circuits 38 may surround the
catheter core as shown in FIG. 5.
[0053] The body of catheter 12 may have a guide wire tube 106
(e.g., a high-density polyethelyne tube) surrounded by an optional
outer tube 108 (e.g., a medium-density polyethylene tube) and a
corresponding tube extension 110. Circuits 38 and array 28 may be
wrapped around marker tube 112 and backing layer 46. If five
circuits 38 are involved, the cross-section of the circuits will
form a pentagon. If four circuits 38 are involved, the
cross-sectional shape will be square. Other numbers of circuits 38
may be used if desired.
[0054] At the input/output 40 of the flex circuit, cable wires 42
may be soldered, welded, or otherwise electrically connected to the
conductors 34 on the flex circuit. The catheter 12 may have a
longitudinal lumen through which the cable 42 passes to equipment
14 (FIG. 1). Plastic tube 118 may be affixed to tube extension 110
using cyanoacrylate adhesive 120. Cyanoacrylate adhesive may also
be used as the adhesive 122 for affixing outer tube 108 and tube
extension 110 to marker tube 112. An ultraviolet-curable adhesive
124 may be used to seal and attach the sensor 16 to the rest of
catheter 12.
[0055] Stiffening member 126 may be used to stiffen the proximal
portion of catheter 12.
[0056] This is merely one suitable arrangement for mounting the
flex circuit and components such as circuits 38 and transducer
array 28 to the catheter 12. Any suitable arrangement may be used
if desired. For example, separate tubes may be provided as unitary
structures. Single tubes or structures may be provided in the form
of individual parts that are affixed using adhesives or other
suitable arrangements. Different types of adhesives and tubing may
be used, etc.
[0057] The backing layer 46 that is used to support the transducer
array 28 preferably has a relatively high acoustic attenuation, so
that acoustic signals propagating radially inward to the center of
the catheter core from transducer array 28 are absorbed. The
impedance of layer 46 is preferably above 3 MRayls, and even more
preferably above 4 MRayls (e.g., 4.2 MRayls). Suppressing spurious
acoustic signals in this way helps to reduce ringing and improves
the signal to noise ratio of the sensor 16. Any suitable material
may be used for backing layer 46. For example, backing layer 46 may
be formed from a mixture of epoxy and hollow microspheres or any
other suitable material that has a high acoustic absorbance. It may
also be advantageous for the backing layer to be formed from an
adhesive so that it may reduce "ringing," as well as attach/secure
the transducer array to the catheter body.
[0058] During the process of forming transducer array 28, an
acoustic matching layer may be formed on each of the array elements
30. The matching layer may be formed, for example, from a 20-30
.mu.m thick layer of material having an acoustic impedance that is
preferably in the range of 5-12 MRayls or more preferably in the
range of 8-10 MRayls. An illustrative matching layer material that
may be used is Eccosorb.TM., available from Emerson & Cuming
Microwave Products, Inc. of Randolph, Mass.
[0059] The matching layer may be disposed between transducer array
elements 30 and flex circuit substrate 32. During operation,
acoustical signals are transmitted from the transducer array
elements through the matching layer and the flex circuit substrate.
The thicknesses (totaling about a quarter-wavelength of the
acoustical signal wavelength) and acoustical impedances of the
matching layer and flex circuit substrate may be selected to
provide good acoustical impedance matching between transducer array
28 and the surrounding tissue or other substances in the patient's
body that are being imaged by the ultrasound sensor 16.
[0060] The quality of sensor 16 may be characterized in terms of
performance metrics such as sensitivity (efficiency), ringdown (the
ability of the transducer to stop vibrating immediately after an
acoustic pulse has been produced by the transducer array),
bandwidth (the ability of the transducer to launch and receive a
wide frequency range of acoustic signals), and cross-talk (the
relative electrical/acoustic isolation of individual elements of
the transducer array from adjacent elements).
[0061] The use of an acoustic matching layer on transducer array
elements 30 improves the ringdown performance of the sensor 16
significantly as compared to configurations in which acoustic
matching capabilities are only provided using the flex circuit
substrate material itself. This is due primarily to the higher
acoustic impedance properties of the matching layer compared to
those available from suitable flexible circuit substrates.
[0062] The acoustic matching layer is used to form an acoustical
antireflection coating that helps couple acoustical signals into
and out of the transducer array 28. Ideally, the acoustic matching
layer would have an acoustic impedance roughly equal to the
geometric mean of the acoustic impedance of the piezoelectric
material of the transducer array 28 (about 31 MRayls) and the
medium in which the catheter is immersed (typically a patient's
blood or other body fluid, which has an acoustic impedance of about
1.5 MRayls). The geometric mean of these two values (given by the
square root of their product) is about 6.8 MRayls.
[0063] The materials that are most suitable for the substrate of
flex circuit 32 are drawn polymer films such as polyimide (Upilex
or Kapton). Such films are flexible enough to roll up into the
cylindrical shape needed for sensor 16 after the electrical
conductors 34 have been formed and the components such as circuits
38 and array elements 30 have been mounted on the flex circuit 32.
However, such polyimide-like films typically have acoustical
impedances of about 3.2 MRayls or less, which is significantly
below the optimum value of about 6.8 MRayls. By using a matching
layer with an acoustic impedance close to 6.8 MRayls (e.g., 5-12 or
6-8 MRayls), the acoustic matching between the transducer array and
the medium in which the catheter is operating (e.g., blood) is
improved, and ringdown (due to reflected energy at the interface
between the device and the medium in which it is operating) is
significantly improved.
[0064] To ensure a high level of uniformity in the matching layer
thickness and to improve the efficiency of the manufacturing
process, the matching layer for all of the elements 30 in array 28
may be deposited and planarized in parallel. The acoustic matching
layer and the other portions of the catheter tip are shown in cross
section in FIG. 6 (taken along the line 6-6 in FIG. 5).
[0065] As shown in FIG. 6, catheter core 18 may have a lumen 24 in
which a guide wire may be disposed during use of the catheter. Core
18 may be solid or may be formed using a hollow tube (e.g., a
hollow plastic tube). If core 18 is a hollow tube, lumen 24 may be
the same size as the inner bore of the tube or may be provided by
nesting a separate hollow tube within the catheter tube. These are
merely illustrative configurations. Any suitable configuration may
be used if desired.
[0066] A backing layer 46 that is highly absorbing to acoustic
waves may be provided on the outer surface of the hollow tube or
cylinder that forms core 18. The transducer array 28 may be
attached to core 18 after backing layer 46. Matching layer 48 is
disposed between the flex circuit 32 and the array 28. The array 28
is shown as forming an annular ring in the drawing of FIG. 6, but
is actually composed of many individual transducer array elements
30 (three of which are shown in FIG. 6). Similarly, the matching
layer 48 is shown as forming a continuous layer of material in the
drawing, but actually lies on top of each of the transducer array
elements 30, as indicated by the three illustrative matching layer
portions 50. The drawing of FIG. 6 is not to scale.
[0067] As described in connection with FIG. 4, the electrodes on
each transducer element are electrically connected to corresponding
electrical conductors 34 on flex circuit 32 prior to installation
of the transducer elements in the catheter. A perspective view of a
block of piezoelectric material 52 that is to be used to form
transducer array 28 is shown in FIG. 7. The piezoelectric block 52
of FIG. 7 has not yet been divided into individual elements 30.
Electrodes 54 and 56 may be formed on block 52 using sputtering,
evaporation, or other suitable deposition techniques and wet or dry
etching or shadow masking or other suitable pattering techniques.
These are merely illustrative methods for forming electrodes 54 and
56. Any suitable techniques may be used to form electrodes 54 and
56 if desired. Electrodes 54 and 56 may be formed from gold (e.g.,
with an underlying adhesion layer of chromium or the like) or any
other suitable metal or conductor.
[0068] When piezoelectric block 52 is installed on flex circuit 32,
the end faces 35 and 36 are electrically connected to conductors 34
on the flex circuit substrate. One suitable technique for
electrically connecting end faces 35 and 36 to conductors 34 is to
use conductive portions such as conductive fillets 58 and 60 (e.g.,
silver paste fillets), as shown in FIG. 8a. (FIGS. 7 and 8a and the
other FIGS. are not drawn to scale. For example, matching layer 48
may be about 20-30 microns thick and piezoelectric block 52 may be
about 66-77 microns thick and 500-1000 microns in length L--i.e., a
14.times. aspect ratio.)
[0069] Conductive portion 58 electrically connects end face portion
35 of electrode 54 to portion 34a of conductors 34. Conductive
portion 60 electrically connects end face portion 36 of electrode
56 to portion 34b of conductors 34. Conductors 34 carry signals
(drive signals for the array elements or acoustic echo signals that
have been converted by the array elements into electrical signals)
between the piezoelectric elements and the electronics in circuits
38 and equipment 14.
[0070] As shown in the lower portion of FIGS. 8a and 8b, matching
layer 48 lies under lower electrode 54, between piezoelectric block
52 and the substrate of flex circuit 32. FIG. 8c shows how fillet
58 is used to form electrical contact with end face 35 and
conductor 34.
[0071] After the electrodes on block 52 have been electrically
connected to conductors 34 as shown in FIG. 8a, 8b, and 8c, the
piezoelectric block 52 may be divided into individual transducer
elements 30, as shown in FIG. 9. Block 52 may be divided into
elements 30 by sawing block 52 (e.g., to leave spaces such as kerfs
62 between each respective pair of elements 30), by scoring block
52 (e.g., using a knife edge), by laser-cutting or
water-jet-cutting of block 52, or by using any other suitable
cutting or dicing scheme.
[0072] The matching layer portions on each transducer element 30
may be formed simultaneously using any suitable technique. One
illustrative approach for forming matching layer 48 on
piezoelectric block 52 involves using template structures such as
template structures 64 of FIGS. 10 and 11. During the manufacturing
process, rectangular blocks of piezoelectric material such as block
52 of FIG. 7 may be inserted into the recessed holes 68 defined in
template layer 66 of structure 64. The matching layer may be
deposited over the piezoelectric blocks. The sides of the holes 68
help to prevent the matching layer material from coating the side
walls of block 52 and ends such as ends 35 and 36. The template
structure 64 of FIG. 11 has larger holes 68, which may be used to
protect the walls of larger blocks of piezoelectric material. Such
larger blocks may then be cut down to form smaller, array-sized
blocks of the type handled by holes 68 of FIG. 10. In the
illustrative examples of FIGS. 10 and 11, only a relatively small
number of holes 68 are shown. In practice, larger number of holes
68 may be provided (e.g., 50-100 or more) to improve the throughput
and economies of scale involved in processing multiple
piezoelectric blocks at a time.
[0073] FIGS. 12-17 show side views of the template structure and
piezoelectric block 52 during processing. As shown in FIG. 12, hole
68 may have a length L that is substantially the same as the
dimension L of the piezoelectric block 52. Template material 66 may
be wax or plastic or any other suitable material for protecting the
sidewalls of piezoelectric block 52. Preferably, template material
66 flows when heated. The template material 66 may have a thickness
T that is about the same as the thickness of the piezoelectric
block. For example, the thickness T may be the same as the
thickness of the piezoelectric block 52 or thickness T may be
thinner than the piezoelectric block thickness by up to about 10
.mu.m. Using a template 66 that is equal or less than the
piezoelectric block in thickness helps to ensure that the tops of
the blocks are well sealed during subsequent top-layer masking
operations. Template material 66 may be supported by a substrate 70
(e.g., a stainless steel substrate or carrier).
[0074] As shown in FIG. 13, each hole 68 in template structure 64
may be filled with a block 52 with an upper electrode 56 and a
lower electrode 54.
[0075] After block 52 has been inserted into the hole defined by
the template 66, a flexible masking member 72 (e.g., a flexible
silicone cover) may be pressed against the upper surface of the
piezoelectric blocks 52 and template portions 66 as shown in FIG.
14. The template structure and blocks 52 may then be heated. This
causes the template material to flow. As the plastic or wax or
other template material of template layer 66 flows, it coats the
side walls of blocks 52. However, the flexible cover 72 seals off
the top layer of block 52, thereby preventing the wax or other
template material from flowing over top electrode 56. By protecting
top electrode 56 in this way, subsequent cleaning operations may be
minimized.
[0076] After the cover 72 has been removed and the template cooled
(optionally, the template may be cooled prior to removal of cover
72), the template layer may appear as shown in FIG. 15. Cusps 74
and other stray template material may be removed from the top
electrode 56 by plasma etching (e.g., in an oxygen plasma) or any
other suitable cleaning procedure.
[0077] As an alternative to flowing, template 66 may be provided
with very tight tolerances such that the template is substantially
the same width L as blocks 52. This allows application of a
matching layer to top electrode 56, while ensuring that the
matching layer does not contact the side wall portions of
electrodes 54 and 56 of blocks 52. As with the flowing approach,
the height of the template may be modified and/or stray template
material may be removed from the top electrode 56 by plasma etching
(e.g., in an oxygen plasma) or any other suitable cleaning
procedure.
[0078] The matching layer 48 may be formed by spreading a suitable
matching layer material on top of the cleaned structures, by curing
the matching layer material, and by polishing the cured matching
layer (if desired). Suitable matching layer materials such as
Eccosorb typically are formed from precursors that have a
paste-like consistency. After curing, the matching layer becomes
solid. Mechanical polishing, chemical-mechanical polishing, or any
other suitable polishing or planarizing technique may be used to
planarize the matching layer portions on top of each of the
piezoelectric blocks 52 in the template structure 64 in parallel. A
side view showing how matching layer 48 may be formed on top of
block 52 and template 66 is shown in FIG. 16.
[0079] After the matching layer 48 has been polished and blocks 52
removed from the template and cleaned in a solvent (if desired) to
remove excess template material from the side walls, each block 52
(now coated with matching layer 48 as shown in FIG. 17) is ready to
be affixed to flex circuit 32 and electrically interconnected with
conductors 32. Blocks 52 may be removed from template 66 by a
variety of techniques, including laser or mechanical cutting.
Alternatively, the template may be thermally cooled and fractured
along interfaces with blocks 52. Further, the template material,
e.g., wax, may be melted to allow easy removal of the blocks from
the template. These are merely illustrative examples. Any suitable
technique for removing blocks 52 having matching layers 48 from
template 66 may be used if desired.
[0080] During the process of coating blocks 52 with matching layer
48, the matching layer on each of multiple blocks 52 (e.g., 50-100
blocks in structure 64) are formed simultaneously (both during the
deposition and curing phase and during the polishing phase).
Preferably, matching layer 48 is segmented from a sheet covering
multiple blocks 52 into individual portions that individually cover
the surface of each block 52 during removal of the blocks from the
template. Such segmentation may be achieved by cutting, cracking,
etc.
[0081] Moreover, each of the transducer elements 30 that is created
when a given piezoelectric block 52 is divided up has a portion of
the simultaneously-formed matching layer 48, which is also divided
up during the dicing of block 52 into elements 30. Because they are
produced in parallel, the simultaneously-formed matching layer
portions on the transducer elements are uniform and economical to
manufacture.
[0082] Illustrative steps involved in fabricating an ultrasonic
imaging catheter 12 having simultaneously-formed matching layer
portions 48 on each of the transducer elements 30 in the array 28
are shown in FIG. 18. At step 76, the patterned template structure
64 may be provided (e.g., by forming a patterned wax or plastic
template 66 on a stainless steel carrier 70).
[0083] At step 78, piezoelectric blocks 52 may be placed into each
of the holes 68 in the patterned template structure 64. The
piezoelectric blocks 52 may be formed from any suitable
piezoelectric material such as lead zirconate titonate composites.
Electrodes 54 and 56 (including end-wall electrode portions 35 and
36) are preferably formed on the blocks 52 before the blocks are
inserted into the holes 68.
[0084] At step 80, the piezoelectric blocks 52 may be covered with
a suitable flexible mask (e.g., a silicone cover sheet). The cover
may be pressed against the top surface of the blocks with
sufficient force to seal off the tops of the blocks (and therefore
electrodes 56).
[0085] At step 82, the template material (and the blocks 52) may be
heated. This causes the template material to flow and coat the
sides of the blocks 52.
[0086] At step 84, the suitable flexible mask may be removed.
[0087] Excess material, e.g., wax or plastic, may be removed by
cleaning the blocks 52 using a plasma etch or other suitable
cleaning technique at step 86.
[0088] At step 88, the matching layer may be formed simultaneously
on all of the blocks 52 in the template structure 64 and on all of
the portions of each block 52 that will later become individual
transducer array elements 30. The matching layer may be deposited
by applying uncured matching layer paste to the surface of the
blocks 52 in template structure 64 and by applying heat and/or
exposing the paste to air or any other suitable ambient environment
to cure the paste.
[0089] At step 90, the cured matching layer 48 may be polished or
otherwise planarized.
[0090] At step 92, the piezoelectric blocks 52 may be removed from
the template structure 64 (e.g., by prying each block 52 from the
structure 64 using a tool such as a knife, by flexing the
substrate, by cooling the substrate and template structure 64
sufficiently to make the wax or plastic of the template 66 brittle
enough to crack along the seams between the template 66 and the
sides of the blocks 52 under pressure, by laser cutting, or using
any other suitable technique). The sides of the blocks 52 will
generally be clean when removed from the template, but an
additional solvent cleaning step or other suitable cleaning
operation may be used to further clean the template material from
the blocks (and from the electrodes on the blocks) if desired.
[0091] If a template structure 64 with larger-sized holes such as
holes 68 of FIG. 11 is being used, the larger sized piezoelectric
blocks 52 may be cut into array-sized blocks at step 94.
[0092] At step 96, an individual block 52 may be attached to flex
circuit 32 (e.g., using a suitable adhesive).
[0093] At step 98, conductive fillets or portions 58 and 60 may be
used to interconnect the ends 35 and 36 of the electrodes 54 and 56
to the conductors 34 of flex circuit 32 (e.g., by applying silver
paste along the interface between the ends of block 52 and the flex
circuit).
[0094] At step 100, the block 52 may be diced, sawed or otherwise
cut or divided into individual transducer array elements 30.
Dividing block 52 into elements 30 also divides
simultaneously-formed matching layer 48, such that each element 30
has an individual portion of matching layer 48.
[0095] If desired, step 100 may also involve coating the back of
the flex circuit (at least in the vicinity of array 28) with a
stabilizing layer of photoresist (e.g., polymethylmethacrylate or
PMMA) or other suitable material, such as Nitto tape. The
stabilizing layer is preferably temporary and may be used as a
sacrificial layer to facilitate dicing and/or formation of the flex
circuit into a cylinder. The dicing process (or other suitable
cutting process) may then be used to cut through both the
piezoelectric block 52 and, optionally, the underlying flex circuit
under kerfs 62 (FIG. 9).
[0096] The cutting process optionally forms cuts 104 through all or
some of the flex circuit 32 between adjacent transducer elements
30, as shown in FIG. 19. The drawing of FIG. 19 shows a sectional
view (i.e., a cross-section across the middle of the transducer
array) taken of the flex circuit 32 in a flattened condition,
before being rolled up to form the cylindrical flex circuit shape
of the sensor 16 that is used during operation of ultrasonic
imaging catheter 12. The drawing of FIG. 20 shows, from an exterior
perspective, how longitudinal cuts 104 (cuts parallel to the
longitudinal axes of the sensor and catheter) are located (e.g.,
evenly-spaced) around the circumference of the distal end of the
cylindrical rolled-up flex circuit that forms sensor 16.
[0097] The cuts 104 (which may be either downward extensions of
kerfs 62 or separate cuts) help to isolate array elements 30 from
each other. The cuts may extend from the edge of flex circuit 32 so
as to leave each array element 30 mounted on its own piece of
substrate material in a diving-board fashion. Alternatively, the
ends of the "diving boards" may be left connected to each other by
making cuts 104 through only the middle portions of the flex
circuit 32 (i.e., the portion of flex circuit 32 between ends 36
and 36).
[0098] The underlying temporary stabilizing layer, which, by virtue
of covering cuts 104, may facilitate the holding of the flex
circuit 32 in the vacuum chucks used during sawing, may be removed
after cutting is complete.
[0099] When the optional flex circuit cutting process is used
during the process of dividing the array 28 into transducer array
elements 30, the transducer array elements become more acoustically
decoupled from each other. This decoupling may increase the width
of the acoustic beam profile associated with each transducer array
element 30.
[0100] Imaging quality is improved when each transducer element
operates relatively independently and has a fairly wide associated
beam profile. When the transducer array elements produce
overly-narrow beams, the beams cannot be combined properly. This
may make it difficult or impossible to sweep the combined beam
through as wide an angle as is desired during the imaging process.
By cutting completely through the portions of the flex circuit that
lie between adjacent transducer array elements 30, the transducer
elements produce wider beams and are less likely to induce
undesirable vibrations in adjacent elements.
[0101] In FIG. 19, flex circuit 32 is shown cut into multiple
pieces. However, as described above and as shown in FIG. 20, cuts
104 may be formed (during step 100 of FIG. 18) as local slits in
the flex circuit in the vicinity of array elements 30, such that
flex circuit 32 remains one piece distal of the array 28.
Furthermore, cuts 104 may be filled with an epoxy or adhesive
having a substantial acoustic mismatch to adjacent transducer
elements 30. The adhesive may comprise, for example, a UV-cured or
heat-cured epoxy. The epoxy facilitates formation of the flex
circuit into a cylinder and provides a fluid seal between elements
30 of the transducer array, while still providing acoustic
decoupling between respective array elements.
[0102] The performance of the ultrasonic imaging catheter 12 may be
improved by using the simultaneously-formed acoustic matching layer
portions on the elements 30 (e.g., the matching layer portions of
Eccosorb formed by parallel-processing the array elements while
still in the form of a unitary piezoelectric block), by using the
high-acoustic-impedance flex circuit substrate (e.g., the substrate
formed of Upilex-S, Kapton, or other material having an acoustic
impedance in the range of 3.5-4.5 MRayls or other suitable range),
by isolating adjacent array element 30 by cutting through the
underlying flex circuit that lies between adjacent elements, or by
using these design approaches in any suitable combination (e.g.,
using only one of these approaches or using any two of these
approaches). Moreover, these aspects of the invention may be used
in any suitable combination with the various other embodiments of
the invention described above.
[0103] It will be understood that the foregoing is merely
illustrative of the principles of this invention, and that various
modifications can be made by those skilled in the art without
departing from the scope and spirit of the invention.
* * * * *